In rotors, the rotor core of a synchronous reluctance electric machine such as electric motor is formed by arranging plurality of core elements into a stack by laminating the core elements together so as to form a stack of consecutive core elements. The core elements are approximately circular, plate-like core elements made of magnetically conductive material, the material such as electric steel having high value of relative permeability.
Each core element of the rotor and therefore also the formed combination i.e. rotor core is operationally divided in sectorial sections. The number of sectorial sections defines the number of poles of the electric motor.
Each of the sectorial sections has at least one magnetic flux barrier. The flux barriers may be a transversal (compared to rotational axis and compared to radius of the rotor core) openings that are axially directed going through the core elements. The flux barriers may be filled with electrically conductive material such as aluminium which has a lower relative permeability than the magnetically conductive basic material (such as electric steel, as stated above) of the core elements.
In synchronous reluctance electric machines and rotors, one can define a d-axis (direct axis) and a q-axis (quadrature axis). Both the d-axis and the q-axis are extending in the direction of the radius of the rotor core, but there is an angle between the d-axis and the q-axis. In a rotor and rotor core, the area with a high magnetic permeability is defining the direction of the d-axis, and the area with a lower magnetic permeability is defining the direction of the q-axis. The torque affecting to the rotor core and to rotor is optimal when magnetic conductivity on d-axis is as high as possible and the magnetic conductivity on q-axis is as low as possible.
In practise, the lower permeability and therefore the lower conductivity on the q-axis area is achieved by the above mentioned transversal flux barriers that may be either empty openings i.e. cut-outs or openings but filled with conductive material having lower magnetic permeability i.e. lower conductivity than the basic material of the core elements.
One aspect relating to synchronous reluctance electric motors and the rotors of those motors is the way how is it is secured that the core elements of the rotor remain firmly together in the stack on consecutive core elements. Therefore, the question relates to the binding means holding the core elements to each other so as to create the tight stack of core elements. This is an important aspect because the rotational speed of the rotor and included rotor core can be several thousand rpm (revolutions per minute). In synchronous reluctance motors, it is a common practice to use axially oriented stud bolts as binding means for holding the core elements together. Those stud bolts are extending through stack of core elements and also though the end plates that are assembled to both ends the stack of core elements. Those stud bolts are equipped with associated tightening means such as nuts.
However, the use of stud bolts or similar structures is not optimal, in regard to costs and the needed manual work. The location of the binding means can also create problems for the electro-magnetic operation of the rotor.
Documents EP1734639, EP2928047 and WO 2012/000561 provide some additional features but still the level of integration is not yet satisfactory.
Therefore, there is a need for a further improvement.